The present invention is directed to a drill particularly a rock drill with a chisel shank and a drill head which has at least one drill bit on a face side pointing in the feed direction. The drill bit comprises at least one cutting edge between a rake surface and a free surface with the rake surface extending before the cutting edge in the direction of rotation of the drill.
European patent application EP 0 452 255 A2 discloses such a drill. The free surface of the drill bit is planar. However, the practical utilization of the drill has shown that the drill feed is unsatisfactory.
The object of the present invention is providing a drill having a tool bit which drill has a higher drill feed.
Given a drill having a tool bit, the object is inventively achieved in that the free surface deviates from a planar surface into the tool bit over at least one sub-area and therefore has a concave portion.
It can be provided that the sub-area, in a sectional view, exhibits a curve that can be described by a progression or a polygonal course. The concave curve thereby results as a limiting case of this polygonal course. It can be particularly provided that the polygonal course or progression is composed of two straight lines.
On the other hand, it can also be provided that the sub-area, in a sectional view, is an elliptical arc. Beneficially, the sub-area, in a sectional view, is a circular arc having a radius Ri. It can be provided that the radius Ri of the circular arc is a function of the thickness S of the drill bit. In particular, it can be provided that the function is Ri=xxc2x7S with x being a range of 1.5 through 2.0. Preferably, x lies in the range of 1.7 through 1.8.
Advantageously, the radius Ri of the circular arc is a function of the nominal diameter D of the drill. In particular, it can be provided that the function is Ri=yxc2x7D, with y being in a range of 0.25 through 0.60. Preferably, y is situated in a range of 0.37 through 0.45.
Moreover, it can be provided that the radius Ri is a function of the driving strength of the drill driving device provided for the utilization of the drill. Furthermore, it can be provided that the radius Ri of the circular arc is a function of the hardness of the material to be drilled with the drill. It must be pointed out at least with respect to the parameters nominal diameter, driving strength and material to be drilled that the radius Ri is not dependent on the individual parameters independently from one another. For example, when a drill having a small nominal diameter is utilized in a powerful drill driving device and soft material, a relatively large radius Ri is necessary for ensuring an optimum drilling feed. On the other hand, when a drill having a large nominal diameter is utilized in a drill driving device of low performance in hard material, a small radius Ri is advantageous.
According to another particular embodiment of the invention, it can be provided that the radius Ri of the circular arc is selected so that the tangent to the circular arc at the location of the cutting edge extends at an angle xcex1xe2x80x2 relative to a plane that is perpendicular to the feed direction, which angle corresponds to the rake angle xcex3. The tangent and the rake surface would form a symmetrical chisel form. The curve of the free surface, which is circular-arcuate instead of planar, is particularly important for increasing the stability of the cutting edge.
According to another embodiment of the invention, it can be provided that the sub-area does not include the cutting edge. Advantageously, a planar surface extends between the sub-area and the cutting edge. This further increases the stability and wearing resistance of the cutting edge.
In addition, it is provided that the planar surface can also follow radially the outer side or end of the sub-area. This assures high stability of the cutting edge in the area in which the greatest distance is traversed.
Advantageously, the planer surface extends at an angle xcex11 relative to a planar surface that is perpendicular to the feed direction. The angle xcex11 lies in a range of 10 through 30 degrees and preferably is 22.5 degrees.
Besides, it can be provided that the sub-area tapers radially towards the outside or outer end. This increases the longevity of the cutting edge particularly in the radially outer area and increases the cutting sharpness.
According to another special embodiment of the invention, it is to be provided that said sub-area is divided into at least two subdivision-areas. Advantageously, the planar surface also extends between the two subdivision-areas. This provides webs for supporting the stability of the cutting edge and the reduction of the risk of breakage.
According to another special embodiment of the invention, it can be provided that the tool bit is fashioned roof-like and has a transverse blade extending between two cutting edges with each cutting edge having a free surface and a rake surface. It is particularly provided that both free surfaces deviate from a planar surface and extend into the tool bit over at least sub-area and are therefore concave.
Beneficially, each sub-area is situated adjacent to the transverse blade. As a result thereof the two outer points of the transverse blade project higher in the feed or bore direction than the center of the blade. Therefore the friction in the center of the main tool bit is reduced vis-a-vis this transition embodiment.
Beneficially, a planar surface follows the side of each sub-area situated relative to the cutting edge. This leadsxe2x80x94as explained abovexe2x80x94to a further increase in the stability of the cutting edge. In addition it is also provided that the planar surface also follows the radially outer edge or end of each sub-area. This also associates with the advantages that have already been mentioned above.
Preferably each of the sub-areas is divided into at least two sub-areas. It can be particularly provided that the planar surface also extends respectively between the at least two subdivision-areas. This improves the stability of the cutting edge particularly with respect to larger nominal diameters or specific cutting edge loads.
Particularly preferred, the sub-area of the one side of the roof-like embodiment is divided at other locations or respectively, at another location than on the other side of the roof-like embodiment formed by the transverse blade. Other location means a radial offset. The purpose of the generated offside is to prevent a xe2x80x9cplacementxe2x80x9d of the planar surface, which serves as free surface reinforcement in front of the face and a reduction of the bore feed.
According to another special embodiment of the invention, a transfer area can be provided on a least one side of the roof-like fashioning forming the transverse blade between the transverse blade and the adjacent sub-area.
It can be particularly provided that this transfer area, toward the transverse blade, passes over into a planar surface.
A transfer area that passes over into a planar surface can also be provided on the radially outer side or end of the sub-area or, respectively, of each sub-area.
Finally, it can be provided that the rake surface or, respectively, each rake surface is planar or concave or convex over at least one sub-area.
The invention is based on the surprising knowledge that a better chisel effect is achieved by means of a specific design of the free surface, whereby this design results in a higher drill feed particularly with respect to rock drills, since, with respect to rock drills, particularly with respect to hammer drills, there is no constant dependency between the bore feed in millimeters per revolution and the number of revolutions due to the pneumatic impact characteristics and conditions. However, the advantages also generally take effect with respect to drills whereby they become particularly effective with respect to drills that are utilized in drilling tools having a higher bore feed. The better chisel effect supplies a higher cutting rate and therefore a higher drilling speed in millimeters per minute.
According to the special embodiments of the invention, as a result of the planar surfaces in the area of the cutting edge and of the planar surfaces for dividing the sub-areas, in which the free surface deviates from a planar surface extending into the tool bit or is a concave depression, the stability of the cutting edge and therefore the service life with respect the inventive drill can be at least made equal or even can be improved compared to traditional drills given a higher bore feed.
Other advantages and features are apparent from the following descriptions of preferred embodiments, the drawings and claims.